The effects of alpha 2 agonists and antagonists on hippocampal theta activity in the freely moving rat.

The alpha 2 agonists detomidine and xylazine were given to freely moving rats implanted with bilateral hippocampal recording electrodes. These alpha 2 agonists produced virtually continuous type 2 (immobility) theta. Type 1 (movement) theta was attenuated with these drugs. Subsequent administration of atropine sulfate eliminated all theta in the hippocampus. Other animals were given the alpha 2 antagonist tolazoline. This drug had no effect on type 1 theta but animals showed an increase in the production of type 2 theta during sensory stimulation. These type 2 theta responses habituated both within and between trials. Tolazoline was ineffective as a blocking agent for the effects of detomidine. Preadministration of atropine sulfate blocked the effects of detomidine on type 2 theta but not on the elimination of type 1 theta. Results are discussed in terms of alpha 2 modulation of the cholinergic and serotonergic systems.